Optical scanners and readers which are designed to read or to score marks, i.e., carbon pencil or ink, on a document commonly rely on the presence or absence of light reflection from the document as an indication of the presence or absence of a mark. Incident light directed onto a document from a light source is typically absorbed by a mark while light contacting the document in the absence of a mark produces a diffused source of reflected light. An optical reading or scanning system employing a line scanning camera including an array of light sensitive devices, i.e., phototransistors, photodiodes, charge-coupled devices, etc., responds to the reflected light and generates an electrical analog output signal which is indicative of the pattern of marks appearing on the document.
A widely-used application of the optical scanner or reader is in the scoring of student's test answer sheets and the totalizing of election ballots. In these applications of the optical scanner the document scanned consists of a preprinted form wherein predefined mark areas are printed with ink which outlines the mark areas within which a carbon pencil or ink mark is to be entered by a student or a voter. Unfortunately, the printing ink defining the mark areas also absorbs light. If, for example, the light source which illuminates the document contains no blue, a blue printed mark area will appear black inasmuch as the other colors present in the light source will be absorbed by the blue ink. A typical technique for avoiding the "ink noise" encountered in optical scanning systems is to use a light source which contains energy outside of the visible spectrum occupied by the various colors of ink which can range in color from violet to red, i.e., 350 to 750 nanometer wavelength.
Unfortunately, the camera lens of a commercially available line scan camera, such as Fairchild Model CCD1300, is normally optimized for use in the visible range. The application of this type of line scanning camera in an optical scanning system is described in detailed pending patent application Ser. No. 000,457, filed Jan. 2, 1979, entitled "Optical Reading System", assigned to assignee of the present invention, and incorporated herein by reference.
Typically, the commercially available line scanning cameras employ lens systems which have poor transmission characteristics and poor resolution for longer wavelengths. Experimental evaluations indicate that the inability of the commercially available camera lens to sharply focus in the near infrared can cause a 0.010 inch vertical line on a 81/2 inch wide paper to appear as a 0.040 inch wide line when viewed through the lens from the image plane. This lens aberration can severely limit the resolution ability of an optical mark scanning or reading system employing a line scan camera.